ISO 8601 / Kibana日期时间转换

I have a datetime pulled from kibana in the format

2016-02-07T04:00:54.090Z

I am trying to understand what the .090 part stands for? This seems to be an almost standard ISO 8601 datetime format (using T and Z) but I cant figure out if the .090 relates to the time (microseconds?), the timezone (..no idea) or some custom value actually unrelated. It could be some sort of combined representation.

I am using DateTime to convert this. So far I have

$dateTime = new \DateTime::createFromFormat('Y-m-d\TH:i:s.???\Z');

I don't think it isn't timezone related (as I assume I would have seen ..Z +01:00 or such) but then I'm not sure if its microseconds since I would have expected the delimeter to be a : rather than a . and more digits